This invention relates to an improvement of the power converter control system for use with a power converter comprising a plurality of rectifier units for converting AC current into DC current.
This type of power converter is suitable for AC electric locomotives as disclosed in, for example, "Brown Boveri Review", December 1973, especially pages 526 to 538 and FIGS. 6 and 7. In such a power converter, in order to prevent an adverse affect upon communication equipment and the like due to high frequency currents of harmonics flowing through the AC section, the DC output section is arranged by a plurality of rectifier units connected in cascade wherein one of the rectifier units is subjected to continuous control of its conduction phase angle and the other rectifier units are subjected to on-off control in their conductivity so that the DC output voltage of the power converter is continuously controlled by a conbination of the continuously controlled rectifier unit and selected ones of the on-off controlled rectifier units.
One example of a circuit for digitally producing changing operation signals applied to the respective rectifier units has been proposed in Japanese patent application No. 81,120/1974 filed on July 17, 1974 and laid open to public on Jan. 27, 1976 as Japanese patent laid open specification No. 10,348/1976.
According to the laid open specification, the changing operation signals applied to the respective rectifier units are produced in the following manner.
Fist, a count output of a reversible counter, which changes the content of count when a phase control-command applied to the continuously controlled rectifier unit reaches a value indicating a maximum or minimum conduction phase angle, is converted into a decimal code by means of a decoder. On the other hand, an output of a master controller indicative of the number of the rectifier units to be operated is compared with the output of the decoder in a comparing and identifying circuit and the content of count of the reversible counter is changed such that both the outputs become identical with each other. By taking into consideration of both the decimal code output converted from the decoder and output of the comparator circuit, changing operation signals to be applied to the respective rectifier units are produced by code-conversion by a code converter.
Thus, the code converter requires a large number of inputs for producing the changing operation signals applied to the respective rectifier units, resulting in complexity of circuit and increase in fabrication process responsible for inefficiency.